Definitions Gene – sequence of DNA that is expressed as a protein (exon) Genes are coded –DNA →RNA→Protein→Trait Transcription – rewritting DNA into RNA which is needed for a gene to be expressed as a protein Operon – a sequence of DNA that will turn transcription on and off

Parts of an Operon Regulatory gene –Repressor Promoter Operator Structural Genes

Parts of an Operon Promoter – site where RNA polymerase attaches to begin transcription Operator – on/off switch –Repressor will attach turning off transcription –If repressor doesn’t fit into operator, RNA polymerase can transcribe structural genes –Transcription occurs

Operator/Repressor Lock n’ Key arrangement

Inducible Operon Transcription is normally off Repressor is active –Fits into operator turning off transcription Inducer –Molecule from the environment –Attaches to the repressor –Changes the shape of the repressor making it inactive.

Inducible Operon –Repressor is active RNA polymerase can not attach to promoter No proteins are translated (synthesized)

Inducer needs to be present to start transcription i.e. inducer is lactose Transcription is on The end proteins produced are enzymes i.e. lactase

Operon is off –Repressor is active –Transcription does not occur Operon is on –Inactive repressor –Proteins (enzymes) are made

Regulatory Gene makes an active repressor

Inducible Animation Lac Operon

Transcription is normally off Repressor is active (fits into operator) Need an outside molecule from environment to inactivate repressor Inducer has the role of inactivating repressor Proteins produced through transcription are enzymes. Enzymes break down inducer. Inducer is absent which makes repressor active again.

Repressible Operon Transcription is normally on Repressor is inactive –Repressor does not fit into the operator CoRepressor is needed to stop transcription –COREPRESSOR is the end protein produced by transcription –Structural protein used by the cell

End product = corepressor

Tryptophan is the corepressor

Repressible Operon Animation Trpytophan Operon

Inactive repressor The build up of the end product turns off transcription Usually makes structural proteins –Proteins needed for cell function When structural proteins are used up by the cell; the corepressor is absent Transcription will turn on again.